1. Field of the Invention
The present invention relates to a normally closed solenoid valve provided with a valve body for opening and closing a flow path of a hydraulic fluid.
2. Background Art
Conventionally, there are two types of solenoid valves for opening and closing a flow path of a hydraulic fluid, one of which is a normally closed solenoid valve that is normally closed and is opened by excitation of a fixed core and a movable core when a current is rendered to flow into a coil, and the other of which is a normally open valve that is normally opened and is closed by excitation of a fixed core and a movable core when a current is rendered to flow into a coil. Such a normally closed solenoid valve is provided with a valve seat for opening and forming a flow path of a hydraulic fluid and a valve body formed at the tip end of a movable core capable of being brought into contact with the valve seat and moving away therefrom. In a demagnetized state, the movable core is seated on the valve body by a spring force of a spring member secured between the movable core and fixed core and is pressed into the direction of closing the valve. In a magnetized state where a current is rendered to flow into a coil, an attracting force works between the fixed core and movable core, and when the attracting force exceeds the spring force of the spring member, the valve body is separated from the valve seat, wherein the flow path of a hydraulic fluid can be opened. Herein, a force in which the attracting force and the spring force working in the direction opposite the attracting force are synthesized or composed is called a “thrust force” working on the movable core.
Such a normally closed solenoid valve is used for a fluid pressure control unit such as, for example, an anti-lock brake unit, etc., in a vehicle. In such a fluid pressure control unit, in order to use a normally closed solenoid valve in a high fluid pressure range, it was necessary to select a spring member having an intensive spring force in order to maintain a valve closed state against a high fluid pressure in a demagnetized state. Accordingly, if a spring member having an intensive spring force is selected, it was necessary to secure a great attracting force in order to secure a thrust force of the movable core against the spring force.
Therefore, the inventor, et al. commenced development of a normally closed solenoid valve that is able to set the attracting force to a greater level by increasing the area of the plane of the movable core opposed to the fixed core. Where a characteristic line S of spacing distance/attracting force is obtained in association with the attracting force thus obtained and the spacing distance between the fixed core and movable core, such a characteristic line as shown in FIG. 3 was obtained. The characteristic lines S1 and S2 of spacing distance/attracting force are based on the normally closed solenoid valve in which the attracting force is set to a greater level, and the characteristic line S3 of spacing distance/attracting force is based on a prior art normally closed solenoid valve. As has been made clear from the characteristic lines shown in the same drawing, in the assembling allowance range from L1 to L2, the characteristic line S3 of spacing distance/attracting force produces almost the same attracting force, and the normally closed solenoid valve in the assembling allowance range produces almost the same initial performance. However, the characteristic lines S1 and S2 of spacing distance/attracting force produce steep slopes, and even in the assembling allowance range, there is a comparatively large difference in the initial performance. That is, a difference will be brought about in the initial thrust force of the normally closed solenoid valve in an assembled state. (See JP-A-11-141723.)